Lubricating oils, such as motor oils, gear oils, hydraulic fluids and transmission fluids typically contain several additives to improve their performance. These can include dispersants, antioxidants, detergents, friction modifiers, de-foaming agents, pour point depressants, and viscosity index improvers.
The viscosities of lubricating oils are temperature dependent, thus as the temperature of the oil is increased the viscosity typically decreases, and conversely as the temperature of an oil decreases the viscosity will increase. A significant loss of viscosity can be detrimental as it may cause wear between engine parts it is designed to protect. A viscosity index improver (VII) is typically a polymeric material, which principally functions by minimizing the viscosity variations over a wide range of temperatures. Normally these are used to reduce the viscosity loss of lubricating oils upon heating.
Random copolymers are commonly used as VIIs. To ensure solubility in a lubricating oil base, these copolymers are formed from at least one monomer whose homopolymer is oil-soluble. Three types of random copolymers commonly used as VIIs are: polymethacrylates (U.S. Pat. No. 6,124,249), olefinic copolymers (US 2003/0073785), and conjugated dienes (U.S. Pat. No. 6,319,881).
Polymers having a controlled architecture, including star copolymers (U.S. Pat. No. 6,034,042) and block copolymers have been described in the art. These polymers can be prepared through a variety of living anionic and living (or controlled) free radical polymerization techniques. These techniques have been used primarily to control the molecular weight distribution.
U.S. Pat. No. 6,538,091 describes a process for the control of a polymer architecture using an atom transfer process (ATRP) based on a redox reaction with a transition metal compound. This process uses an initiating system resulting in a copolymer having a predictable molecular weight and a controlled polydispersity. Polymers made by the process are described as useful for molding materials, barrier materials, thermoplastic elastomers, and amphiphilic surfactants. This controlled radical polymerization technique has several drawbacks such as, residual metallic by-products which can be detrimental to many applications (for example see U.S. Pat. No. 6,610,802) and limitations in polymer composition. Furthermore, the reference does not describe the use of any of the copolymers in lubricating oils.
Random copolymers made by ATRP have been used as pour point depressants (U.S. Pat. No. 6,391,996), and viscosity index improvers (US2002/0188081). The '081 reference mentions that the ATRP process could be used for blocky copolymers, but fails to exemplify such a use, or recognize the large VII benefit of using such block copolymers in lubricating oils. Also, gradient copolymers synthesized by ATRP have been shown useful as pour point depressants in U.S. Pat. No. 6,403,745. Again, the use of relatively high catalytic amounts of metal compounds leads to product containing residual metal contamination. These metallic by-products are detrimental in engine-type lubricant applications and require removal, which is difficult and requires laborious procedures.
The use of multifunctional lubricant additives has been described in U.S. Pat. No. 6,319,881.
Block copolymers have also been shown to be useful as VIIs. Block copolymers of a vinyl aromatic monomer and a vinyl aromatic-co-acrylic block prepared by stabilized free radical polymerization are described in U.S. Pat. No. 6,531,547. These patents describe the use of TEMPO-based nitroxide derivatives for the synthesis of the corresponding block copolymers. This class of free radical control agent does not provide control over acrylic type monomers. Specifically, the use of methacrylics will lead to side and termination reactions such as disproportionation, which inhibits the formation of block copolymers and long chain molecules (as described by Ananchenko et. al. in the Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 40 pp 3264-3283). Also, block copolymers of ethylene and alpha-olefins have been described in US 2003/0073785 and block copolymers of poly(conjugated dienes) and poly(monovinyl aromatic hydrocarbons have been described in U.S. Pat. No. 6,303,550. None of the above references makes use of a controlled architecture copolymer having at least one pure acrylic block segment for use as a VII.
U.S. Pat. No. 5,002,676 describes the preparation of block copolymers containing selectively hydrogenated conjugated dienes and t-butyl methacrylate. U.S. Pat. No. 6,350,723 teaches the synthesis of block copolymers through the living anionic polymerization of a conjugated diene and an alkyl methacylate monomer. These references exemplify the use of block copolymers containing conjugated dienes and hydrogenated dienes, but fail to teach the specific copolymers of the present invention. Also these references do not teach the significance of tailoring block solubilities or allow for the formation of gradient compositions. Furthermore, living anionic polymerization suffers from several drawbacks, such as, ineffectiveness at temperatures above −20° C., poor copolymerization between polar and non-polar comonomers, and the inability to use monomers that can be easily deprotonated. Therefore functional monomers cannot be incorporated, and the copolymerization of monomer mixtures can be problematic and/or unusable. Furthermore this process can be expensive and difficult or impractical to carry out on an industrial scale as bulk or emulsion techniques cannot be used, extremely pure reagents are necessary (even trace amounts of protic material inhibits polymerization), and an inert atmosphere is requisite.
A process for preparing copolymers in the presence of a stable free radical from the nitroxide family is described in U.S. Pat. No. 6,255,402. Nitroxide-mediated stable radicals have been used to produce controlled block copolymers, as described in U.S. Pat. No. 6,255,448, and US 2002/0040117. These references, incorporated herein by reference, do not describe the use of the copolymers in lubricating oils.
Surprisingly it has now been found that an acrylic block copolymer formed by a controlled radical polymerization, produces excellent viscosity index improvement in lubricating oils. The polymers of the invention produce a greater VI improvement than found in random copolymers or other block copolymers currently used. While the properties attained in traditional copolymers are typically an average of the properties imparted by the resultant monomers incorporated, block copolymers lead to a material containing the characteristic properties inherent to the parent homopolymers comprising each segment. Therefore, the use of block copolymers is particularly adventitious for the formation of materials containing multifunctional properties. Furthermore, this class of polymers should provide enhanced shear stability due to the selected monomer composition, the controlled molecular weights, and molecular weight distribution provided by the controlled polymerization process. The viscosity modifying advantages of these copolymers for lubricant oil applications can be exemplified by the excellent performance demonstrated in typical SAE Standard J300 viscosity classification testing and ASTM D 2270 testing. Furthermore these block copolymers can be used to thicken any number of oil-based compositions.